The present invention relates to electrical energy storage devices such as advanced supercapacitors and batteries and, more specifically, to such devices that use polymer modified electrodes.
Secondary current sources (storage batteries) make it possible to accumulate, store and give up electric power to an external electric circuit. Among these are conventional batteries, conventional capacitors and electrochemical capacitors (also called Supercapacitors or Ultracapacitors)xe2x80x94[B. E. Conway, Electrochemical Supercapacitors// Kluwer Acad. Plen. Publ., NY, 1999, 698 p.].
A conventional electrochemical supercapacitor usually includes a hermetically sealed housing filled with electrolyte, a positive electrode (anode) and negative electrode (cathode) placed inside said housing, a separator that separates anode space from cathode space and special lead terminals connecting the supercapacitor to external electric circuits.
Electrochemical supercapacitors are based on the capacitive (not battery type) or Faradic (battery type) method for storing electric power. In the capacitive type supercapacitors, the capacity of the double electric layer formed at the electrolyte/electrode boundary is used for accumulating energy. Carbon materials having a large specific surface are usually employed as the electrode in such supercapacitors. No chemical or phase changes take place on the electrode surface or in the electrode space during the charge/discharge process in such a device.
In Faraday type supercapacitors, the charge/discharge process is accompanied by redox reactions on the electrode surfaces. In contrast to conventional batteries, these processes take place in a thin layer of electrically active substance on the electrode surface. The surface of electrodes in many known supercapacitors of this type is covered with metal oxides.
Both above mechanisms of energy accumulation exist in known energy storage devices, which are usually classified by the mechanism that makes the major contribution to the energy accumulation and storage process. Electrochemical supercapacitors have very high specific power (as high as 10 kW/kg) and long service life (up to 1 million charge/discharge cycles). These features open a wide range of potential applications for electrochemical supercapacitors [Supercapacitor Market Survey, World Markets, Technologies and Opportunities: 1999-2004 Technical-Economic Analysis for 2000, Tyra T. Buczkowski, ISBN#1-893211-05-32].
Nevertheless, known electrochemical supercapacitors are not free from disadvantages. In particular, they have low specific energy capacity. The value of specific energy capacity for commercially available electrochemical supercapacitors lies within the relatively low range of 1-10 Wxc2x7h/kg.
The highest value of specific energy capacity was claimed for electrochemical supercapacitors of Faradic type that include carbon electrodes with ruthenium oxide on their surface. It is around 30 Wxc2x7h/kg [U.S. Pat. No. 6,383,363]. However, high cost of ruthenium would impede the wide application of such devices.
The maximum values of specific energy capacity of known supercapacitors are limited primarily by the nature of materials used for electrode manufacturexe2x80x94i.e. metal oxides. Metal oxides require supplement of conductive additives, which increase the weight of the system and, therefore, reduce the specific energy capacity. These materials also contribute to the high cost of these devices.
Several attempts have been made to obtain fundamentally new materials and technologies for the design and manufacture of electrochemical supercapacitors. These attempts include chemical modification of electrodesxe2x80x94for example, by immobilizing conducting polymers on the inert electrode surface.
Conducting polymers are subdivided into two groups [B. E. Conway, Electrochemical Supercapacitors// Kluwer Acad. Plen. Publ., NY, 1999, 698 p]:
1) The so-called xe2x80x9corganic metalsxe2x80x9d or conducting polymersxe2x80x94these are polymers with a conduction mechanism similar to that of metals;
2) Redox polymersxe2x80x94i.e. compounds in which electron transfer is effected mainly due to redox reactions between adjacent fragments of polymer chain.
Polyacetylene, polypyrrole, polythiophene and polyaniline represent examples of xe2x80x9corganic metalsxe2x80x9d. In partially oxidized form, these polymers offer an even greater degree of conduction, and they can be considered as salts consisting of positively charged xe2x80x9cionsxe2x80x9d of polymer and counter-ions evenly distributed over its structure (these counter-ions support the overall electrical neutrality of a system).
The polaron theory of conduction is acknowledged to be the main model of charge transfer in conducting polymers [Charge Transfer in Polymeric Systems //Faraday Discussions of the Chemical Society. 1989. V.88]. In solid state physics, a polaron is a cation radical which is partially delocalized over a polymer fragment. The polaron becomes stable, thus polarizing its environment. (#Paragraph 1)
xe2x80x9cOrganic metalsxe2x80x9d can be produced by electrochemical oxidation of appropriate monomers on an inert electrode surface. These polymers can be converted from a conducting state (i.e. oxidized state) into a non-conducting state (i.e. reduced state) through variation of the electrode potential. Transition of a polymer from the oxidized state into the neutral reduced state is accompanied by the egress of charge-compensating counter-ions from the polymer into the electrolyte solution, in which the process is conducted. The reverse is also possible.
Both purely organic systems and polymer metal complexes (i.e. metal organic compounds) fall into the category of redox polymers [H. G. Cassidy and K. A. Kun. Oxidation Reduction Polymer //Redox Polymers. Wileyxe2x80x94Interscience, New York, 1965]. Polymers containing metals are better conductors than those without.
As a rule, polymer metal complex compounds are produced via electrochemical polymerization of source monomer complex compounds with octahedral or square-planar configurations, wherein electrochemical polymerization being performed on inert electrodes. As will be shown below, the spatial configuration of monomers plays a crucial role in the formation of polymer structures suitable for use in supercapacitor. Polypyridine complexes of composition poly-[Me(v-bpy)x(L)y], where:
Mexe2x95x90Co, Fe, Ru, Os;
v-bpy=4-vinyl-4xe2x80x2-methyl-2,2xe2x80x2-bipyridine;
L=v-bpy (4-vinyl-4xe2x80x2-methyl-2,2xe2x80x2-bipyridine), phenanthroline-5,6-dione, 4-methyl phenanthroline, 5-aminophenanthroline, 5-chlorophenanthroline; (x+y=3) represent an example of redox polymers formed using octahedral source complex compounds [Hurrel H. C., Abruna H. D. Redox Conduction in Electropolymerized Films of Transition Metal Complexes of Os, Ru, Fe, and Co //Inorganic Chemistry. 1990. V.29. P.736-741].
Metal ions that may be in different states of charge represent redox centersxe2x80x94i.e. atoms participating in redox reactions in a polymer. Metal complexes having only one possible state of charge (zinc, cadmium) do not produce redox polymers. Conduction of redox polymers requires the presence of a branched system of conjugated Π-bonds that serve as conducting xe2x80x9cbridgesxe2x80x9d between redox centers in a ligand environment of complexes. When a redox polymer is completely oxidized or completely reduced (i.e. all its redox centers are in one state of charge), charge transfer along the polymer chain is impossible and redox polymer conduction is close to zero. When redox centers are in different states of charge, exchange of electrons is possible between them (this proceeds in the same manner as in solution in the course of redox reactions). Therefore, conduction of redox polymers is proportional to the constant of electron self-exchange between redox centers (kco) and to concentrations of oxidized [Ox] and reduced [Red] centers in a polymer. In other words, the redox polymer conduction is xcx9ckco[Ox] [Red].
Conduction of redox polymers is maximum when the concentration of oxidized redox centers is the same as the concentration of reduced redox centers, which corresponds to the redox system having a standard redox potential Eo([Ox]/[Red]). Because redox centers of polymers, which are based on coordination compounds, may be in different states of charge these redox polymers are called xe2x80x9cmixed-valence complexesxe2x80x9d or xe2x80x9cpartially oxidized complexesxe2x80x9d.
Transition of redox polymer molecules from the oxidized state to the reduced state is accompanied (as has been described for conducting polymers) by the egress of charge-compensating counter-ions from a polymer into the electrolyte solution, in which the process is conducted, and vice versa.
There are known electrochemical supercapacitors with electrodes modified by xe2x80x9corganic metalsxe2x80x9d (conducting polymers)xe2x80x94[B. E. Conway, Electrochemical Supercapacitors// Kluwer Acad. Plen. Publ., NY, 1999, 698 p.]. Application of xe2x80x9corganic metalsxe2x80x9d in electrochemical supercapacitors has demonstrated a number of essential advantages offered by these materials over other systemsxe2x80x94in particular, over metal oxides:
1. The polymers possess intrinsic conduction, which makes it unnecessary to use dispersion current supply matrices;
2. Polymer materials are at least one order of magnitude cheaper than the majority of oxide materials used in supercapacitors;
3. Formation of conducting polymer materials can be carried out directly on the electrode surface without using a number of intermediate synthetic steps;
4. Conducting polymer materials enable one to create supercapacitors of both known typesxe2x80x94capacitive supercapacitors or xe2x80x9cdouble-layerxe2x80x9d supercapacitors (based on the charge/discharge processes of a double layer of electrodes) and xe2x80x9cpseudo-capacitivexe2x80x9d supercapacitors or xe2x80x9cFaradicxe2x80x9d supercapacitors (based on the oxidation/reduction processes of an electrically active substance immobilized on the electrodes);
5. Although inferior to oxide materials in terms of stability, polymers nevertheless make it possible to create energy-storage systems with a long service life (up to 105-106 cycles).
There are three types of polymer-based supercapacitors [B. E. Conway, Electrochemical Supercapacitors //Kluwer Acad. Plen. Publ., NY, 1999, 698 p; U.S. Pat. No. 5,527,640].
Type I: both electrodes are made of one and the same polymer. When in a completely charged state, one electrode is oxidized completely, while another electrode is in the uncharged (neutral) state. In this case, the potential difference is usually 0.8-1.0 V. As a result of discharge, the final voltage of the supercapacitor, as a rule, is no greater than half of the indicated valuexe2x80x94i.e. 0.4-0.5 V [B. E. Conway, supra, p.319].
Such a change in voltage between the electrodes (to a half of the difference of potentials in the beginning of discharge cycle) of supercapacitor is characteristic for systems in which the capacity is determined by the capacity of the double layer of electrodes (so-called xe2x80x9cdouble-layer supercapacitorsxe2x80x9d).
Type II: in principle, supercapacitors of this type are similar to those of type I. However, polymers having different redox potentials are used in the supercapacitors of this type in order to increase the difference of potentials [B. E. Conway, supra, p.320].
Type III: one electrode is made of the oxidized form of a polymer, while the other electrode is made of a reduced form of the same polymer [B. E. Conway, supra, pp.320-321]. Supercapacitors of Type III offer the highest values of voltage (up to 3 V when using non-aqueous solvents), and, hence, the highest specific energy capacity. Supercapacitors of Type III are considered to be the most promising electrochemical supercapacitor systems.
According to B. E. Conway, supercapacitors based on conducting polymers with the metallic type of conduction may be considered as xe2x80x9cdouble-layerxe2x80x9d systems, accumulating energy due to charging of the polymer surface and compensation for the acquired charge by counter-ions that are present in the electrolyte. The specific energy capacity of such systems is mainly determined by the degree of development of conductive substrate surface; as a rule, it is not high. Conway [B. E. Conway, supra, p.321] gives the following values of specific energy accumulated by polymers for the three types of polymer-based supercapacitors (see Table below):
As can be seen from this table, specific energy capacity of Type III electrochemical supercapacitors with electrodes modified by an xe2x80x9corganic metalxe2x80x9d (conducting polymer), is higher than that of supercapacitors with electrodes with metal oxides on their surface. However, the difference is not large.
As compared to electrodes modified by xe2x80x9corganic metalsxe2x80x9d (conducting polymers), redox polymers and electrodes with redox polymers on their surface potentially offer higher specific energy capacity owing to the greater contribution of the Faradic component of capacity to the overall capacity of the polymer, which is associated with multi-electron oxidation/reduction of metal centers.
Nevertheless, improving supercapacitor design and performance, especially as it concerns the increase in specific energy, still remains very important problem.
Further details on prior known elements, processes, and devices related to the field of the present invention can be further understood from the following references:
1. U.S. Pat. No. 5,729,427
2. U.S. Pat. No. 6,383,363
3. U.S. Pat. No. 5,840,443
4. B. E. Conway supra p. 319, 320, 321
5. P. Audebert, P. Capdevielle, M. Maumy. Redox and Conducting Polymers based on Salen Type Metal Units; Electrochemical Study and Some Characteristics// New J. Chem. 1992. V. 16 P. 697
It is an object of the present invention to produce an energy-storage devicexe2x80x94e.g., an electrochemical supercapacitorxe2x80x94having electrodes modified by redox polymer complex compound of transition metal formed of stacked transition metal complex monomers.
Design of electrodes represents a principal feature of the electrochemical capacitor according to the present invention, where at least one of the electrodes includes a polymer-modified conductive surface, where the polymer comprises stacked transition metal complex monomers.
It is a further object of the present invention to provide an electrochemical supercapacitor with higher specific energy capacity than known and reported prior devices.
One exemplary electrochemical capacitor, according to the principles of the present invention, includes a polymer-modified electrode having a conductive substrate, on which a layer of energy-accumulating redox polymer is applied, with said redox polymer being a stack-type polymer complex compound of a transition metal, which has at least two degrees of oxidation. The stack-type polymer complex compound is comprised of monomer fragments of planar structure having a branched system of Π-bonds. Preferably, the deviation from a plane is no greater than 0.1 nm. The thickness of energy-accumulating redox polymer layer is preferably from 1 nm to 20 xcexcm. The polymer complex compound may take a number of forms, examples of which appear in the detailed description below.
The set of features according to the invention disclosed herein provides for the creation of a highly effective supercapacitor due to the application of the electrode chemically modified by a redox polymerxe2x80x94polymer metal complex with substituted tetra-dentate Schiff""s base. This results in a dramatic increase in the specific energy capacity of a supercapacitor over known supercapacitors. The possibility of creating electrochemical supercapacitors of Type III offering the highest energy capacity represents very important issue. The polymer used for electrode according to the present invention is capable of making a transition both into the oxidized state and reduced state. In other words, the polymer is capable of operating both on a positive electrode and negative electrode enabling the potential of one example of an electrochemical electrode to go as high as 3 V and the value of the specific energy capacity of the polymer to be above 300 J/g.
A feature of the electrode, according to the principals of the present invention, for energy storage devices includes a new combination of a layer of energy accumulating redox polymer on the conductive substrate of the electrode, with said redox polymer being a stack-type polymer complex compound of a transition metal that has at least two degrees of oxidation, which is comprised of monomer fragments of planar structure with preferably a deviation from a plane of no greater than 0.1 nm and having a branched system of Π-bonds.